Digital inkjet printing by means of drop-on-demand print heads is now a commonly used application in industry. A wide variety of inkjet printer variants for desktop and office printers has been known in the art for many years. However, the print heads used in industry are differently designed to those for desktop and office printers and typically also consume substantially more ink due to the large areas to be printed. The printing of bottles, for example, can generate an annual consumption of several tons. A suitable ink supply is therefore an essential component of printing systems of this kind and has a crucial influence on the quality of the print result and profitability.
Drop-on-demand print heads have a small ink chamber behind each nozzle, which is filled with the ink medium. The ink medium is ejected through the nozzles of the print head by a short-term increase in pressure being generated in said ink chamber using various methods. In the method known from EP 0 352 978 A2, a minimum part of the ink medium is heated by heating in the ink chamber, so that part of the ink medium is evaporated. The resulting expansion generates a pressure pulse.
In the method described in U.S. Pat. No. 5,124,716 A a pressure pulse is generated in the ink chamber of the print head via a piezoelectric crystal. This pressure pulse causes a drop of the ink medium to be pressed through the nozzle and therefore ejected.
The print heads, which usually have about a thousand nozzles, require there to be a negative pressure inside the ink chamber upstream of the nozzles, since otherwise the low-viscosity inks would leak out of the nozzles, despite the nozzle openings being small, only micrometres in size. This negative pressure is in the order of roughly −10 mbar.
Two different types of print heads are generally used in industry. One works as a dead-end shooter in which the ink medium is simply applied to the print head and the supply is achieved via the geodetic height (cf. U.S. Pat. No. 4,638,337 A). This means that the ink medium is conveyed to the print head and said print head with its ink chambers for the ink medium forms a dead end until the ink is ejected through the nozzles. The pressure inside the ink chambers of the print head substantially corresponds to the pressure at the inlet of the print head in this case. The negative pressure in the print head may cause air to be drawn into the print head through the nozzles. The air bubbles in the ink chamber resulting from this mean that the volume in the ink chamber is compressible, as a result of which a pressure build-up for emitting a drop is not possible in a reproducible manner.
The second possibility is the so-called recirculation method, as is known from U.S. Pat. No. 5,818,485 A, for example. In this case, the ink is repeatedly pumped through the print head so that there is a continuous flow of ink through the print head. In this way, the air drawn through the nozzles can, where necessary, be removed from the ink chamber once again.
These print heads were originally designed for planographic printing, in which the surface is to be printed lies in a horizontal plane and the ink is applied from top to bottom, as is the case with typical office printers were. Meanwhile, the print heads are also used in a vertical arrangement, however. This has the advantage that when printing three-dimensional objects the side surfaces can also be printed, without the side to be printed being moved into a horizontal position. When printing bottles, for example, they would otherwise have to be turned over for a very short time (about 1/10 second) and then realigned.
However, the vertical arrangement requires a completely different precision in the ink supply. The print heads themselves typically have a printing length via the nozzles, in other words a distance between the lowermost and uppermost nozzle, of about 70 mm. Therefore, the pressure difference due to gravity within the nozzles of a vertically disposed head, assuming a density of 1000 kg/m3, is only 7 mbar. The lowermost nozzle therefore has an ink supply pressure that is 7 mbar higher than the uppermost nozzle. At a nominal pressure of −10 mbar there is no longer any scope for tolerance. Any pressure fluctuation can therefore be decisive as to whether the quality is sufficient for good printing.
Known recirculation systems have a first tank from which the ink is supplied to the head at positive pressure by means of a pump, and a return pump which generates a negative pressure at the ink outlet of the print head which is high enough to generate the negative pressure described in the print head due to the flow resistance in the print head.
In this case, pumps denote machines for conveying substantially incompressible fluids, such as an ink medium, for example. Pumps are to be distinguished from compactors and compressors which are machines for compressing and conveying gases and gaseous media.
The pressure prevailing within the print head upstream of the nozzles is referred to as the meniscus pressure which corresponds to the nominal pressure in the print head and which, as described, should be a slight negative pressure. Also important for the function of the print head is the pressure difference between the feed and return which is responsible for the flow rate, in other words the throughput of ink through the print head.
In order to maintain a uniform pressure, the ink supplies known in the art work with pumps for the ink and pulsation dampers. These systems have a highly complex design and elaborate control. In this case, despite the smoothing by the pulsation dampers, pressure spikes can occur which severely affect the print quality.